A display device using a light-emitting element (organic EL element) in which an organic compound is used as a light-emitting substance has been developed rapidly as a next generation lighting device or display device because it has advantages that such a light-emitting element can be manufactured to be thin and lightweight, has very high response speed with respect to an input signal, and has low power consumption.
In an organic EL element, when a voltage is applied between a pair of electrodes between which a light-emitting layer is interposed, electrons and holes are injected from the electrodes. The injected electrons and holes are recombined to form an excited state of a light-emitting substance contained in the light-emitting layer, and when the excited state relaxes to a ground state, light is emitted. A wavelength of light emitted from a light-emitting substance is peculiar to the light-emitting substance; thus, by using different types of organic compounds as light-emitting substances, light-emitting elements which exhibit various wavelengths, i.e., various colors can be obtained.
In a case of a display device which is expected to display images, such as a display, at least three colors of light, i.e., red, green, and blue are required to be obtained in order to reproduce full-color images. In the case of a lighting device, in order to obtain high color rendering property, light having wavelength components thoroughly in the visible light region is ideally obtained. Actually, two or more kinds of light having different wavelengths are mixed to be used for lighting application in many cases. Note that it is known that by mixing light of three colors, red, green, and blue, white light emission having high color rendering property can be obtained.
Light emitted from a light-emitting substance is peculiar to the substance, as described above. However, important performances as a light-emitting element, such as lifetime or power consumption, are not only dependent on a light-emitting substance but also greatly dependent on layers other than a light-emitting layer, an element structure, properties of the light-emitting substance and a host, compatibility between them, or the like. Therefore, it is true that many kinds of light-emitting element materials are necessary in order to show the growth of this field. For the above-described reasons, light-emitting element materials which have a variety of molecular structures have been proposed (for example, see Patent Document 1).